I don't follow baseball so please correct me if this information is for something less than it seems to me. A quick google search brought up two services (MLB.tv and MLB Extra Innings). The former seems to cost about $40-$50 while the latter is in the $200 range for the season. Also, Extra Innings looks like an add on package to an existing cable/satellite subscription while MLB.tv is a standalone service.
MLB.tv: http://mlb.mlb.com/mlb/subscriptions/index.jsp
Extra Innings (found price info on Wikipedia): http://en.wikipedia.org/wiki/MLB_Extra_Innings